This invention relates to welding, and, more specifically, to heat treating weld beads during the welding process.
Metal parts are commonly fixed together by welding, which involves locally melting the parts to effect a solidified weld joint or bead therebetween. Welding may be effected with or without a filler material at the weld joint.
Since the parent material or substrate is necessarily melted during welding, the metallurgical properties of the substrate are correspondingly affected. The affected region includes a heat-affected zone around the weld joint which, although not melted during the welding operation, nevertheless experiences a high temperature which alters the metallurgical properties of the parent metal material in that zone.
The parts may be formed of various metal alloys whose metallurgical properties and microstructures are affected differently at the weld bead and heat-affected zone. In some alloys, the material strength around the weld bead is reduced by the welding process which shortens the useful life of the welded parts by subjecting them to premature material cracking near the weld beads.
In order to improve the material properties at a weld joint, the parts may be preheated prior to welding, or they may be postheated after the welding, or both, to improve the microstructure and strength within the weld and the heat-affected zone of the weld.
The weld may be further improved by minimizing the amount of heat applied to the parts during welding and limiting the extent of the heated zone.
Various types of welding are known in the art and have different advantages and disadvantages. Some common examples include electrical arc welding, laser beam welding, and electron beam welding. Electrical arc welding has high heat input capability and is commonly used for welding relatively thick metal parts along a weld prep or groove which is filled using a welding filler material in multiple passes of overlying weld beads. The heat-affected zone is correspondingly relatively large.
Laser and electron beam welding are commonly used for limiting the extent of the heat-affected zone and effecting precise, narrow welds, with better control of the heat-affected zone.
Welding is further complicated by the nature of the welded parts and their intended use. For example, boiling water or pressurized water nuclear reactors include pressure vessels in which water is circulated for cooling the reactor core. The radioactive and high temperature environment is hostile and requires specialized metal alloys for the various components of the pressure vessels for obtaining enhanced useful lives.
Although corrosion resistant alloys are used in nuclear reactors of the type indicated above, they may nevertheless be subject to corrosion over their useful life which may be limited if stress corrosion cracking should occur. Stress corrosion cracking is a known problem in nuclear reactor components such as pipes exposed to high temperature water during operation. Stress corrosion cracking can be avoided by using special metal alloys and carefully controlled welding to limit residual stresses in, and thermal sensitization of, the weld.
Since the reactor pipes are generally thick-walled components, electrical arc welding with a suitable filler material is typically used for joining the pipes. The weld preps or grooves are preferably made as narrow as practical for minimizing the resulting heat-affected zone. Weld preheating and postheating may also be used for reducing undesirable microstructure changes near the weld bead. However, in view of the practical problems, including size, configuration, and available space for welding nuclear reactor components such as pipes, welding, preheating, and postheating are presently limited in their ability to produce optimum welds.
Accordingly, it would be desirable to provide an improved welding and heat treating process for improved weld performance.